Epoxy resin compositions have been widely used in a wide range of applications for their temperature resistance and mechanical properties such as tensile and flexural properties. Epoxy resin compositions are particularly useful for fiber-reinforced composites. Fiber-reinforced composites comprise cured epoxy resins as a polymer matrix with fibers (for example, glass fibers and carbon fibers) embedded therein.
Many applications for fiber-reinforced composites require a cured epoxy resin having high thermal resistance, that is, a glass transition temperature (Tg) of 160 degree Celsius (° C.) or higher. It is thus desirable to simultaneously increase mechanical properties such as tensile properties and flexural properties, while maintaining or increasing thermal resistance. In particular, it is desirable for the tensile elongation property of the cured epoxy resin to be high enough so that the polymer matrix formed therefrom will not break, before the fibers break which are embedded within the polymer matrix. However, increasing tensile elongation typically tends to decrease thermal resistance.
In addition, components in epoxy resin compositions must be compatible with each other to remain as a homogeneous composition throughout processing. A typical process for manufacturing fiber-reinforced composites comprises the steps of: loading a batch of an epoxy resin composition into a resin bath, drawing fibers through the epoxy resin composition in order to impregnate the fibers with the epoxy resin, and curing the resin impregnated in the fibers to form a fiber-reinforced composite. A typical residence time for an epoxy resin composition in a bath during processing can be eight hours or more at a processing temperature of from 20 to 45° C. The epoxy resin composition should remain a homogeneous mixture throughout that residence time. Thus, the components of the epoxy resin composition must be sufficiently compatible so as to remain homogeneous (that is, avoid phase separation) during the residence time in the bath. Incompatibility between the components of the epoxy resin composition results in phase separation during residence in the bath, typically causing the resulting fiber-reinforced composite to have one or more of the following undesirable properties: rough surface, inconsistent properties and/or a reduction of mechanical properties such as tensile and flexural properties.
Therefore, it is desirable to provide a curable epoxy resin composition, wherein the composition upon curing increases one or more of tensile properties, flexural properties and Tg without concomitantly decreasing any of those properties significantly relative to a conventional technology. It is also desirable that the components of a curable epoxy resin composition have sufficient compatibility with each other so that the composition exhibits no phase separation for at least eight hours at 25° C. after mixing.